Flat, box-like muffler configurations, such as shown in U.S. Pat. No. 4,700,806, issued October 20, 1987, are at increased risk of failure from the action of corrosive liquids collecting on flat exterior and interior surfaces. Water from wet and snowy pavements, for example, and even road salt may collect in certain areas on top of the muffler and rapidly corrode the metal. Acidic liquid that condenses from the exhaust gases inside the muffler may collect on flat bottom surfaces and produce premature corrosive failure.